The Defense Waste Processing Facility will be used to immobilize Savannah River Site high-level waste into a stable borosilicate glass for disposal in a geologic repository. Proper control of the melter feed composition in this facility is essential to the production of glass which meets product durability constraints dictated by repository regulations and facility processing constraints dictated by melter design. A technique has been developed which utilizes glass property models to determine acceptable processing regions based on the multiple constraints imposed on the glass product and to display these regions graphically. This system along with the batch simulation of the process is being used to form the basis for the statistical process control system for the facility. 13 refs., 3 figs., 1 tab.
The Defense Waste Processing Facility will be used to immobilize Savannah River Site high-level waste into a stable borosilicate glass for disposal in a geologic repository. Proper control of the melter feed composition in this facility is essential to the production of glass which meets product durability constraints dictated by repository regulations and facility processing constraints dictated by melter design. A technique has been developed which utilizes glass property models to determine acceptable processing regions based on the multiple constraints imposed on the glass product and to display these regions graphically. This system along with the batch simulation of the process is being used to form the basis for the statistical process control system for the facility.
Radioactive high level waste (HLW) at the Savannah River Site (SRS) has successfully been vitrified into borosilicate glass in the Defense Waste Processing Facility (DWPF) since 1996. Vitrification requires stringent product/process (P/P) constraints since the glass cannot be reworked once it is poured into ten foot tall by two foot diameter canisters. A unique "feed forward" statistical process control (SPC) was developed for this control rather than statistical quality control (SQC). In SPC, the feed composition to the DWPF melter is controlled prior to vitrification. In SQC, the glass product would be sampled after it is vitrified. Individual glass property-composition models form the basis for the "feed forward" SPC. The models transform constraints on the melt and glass properties into constraints on the feed composition going to the melter in order to guarantee, at the 95% confidence level, that the feed will be processable and that the durability of the resulting waste form will be acceptable to a geologic repository.
The Defense Waste Processing Facility at the Savannah River Site (SRS) will vitrify high-level nuclear waste into borosilicate glass. The waste will be mixed with properly formulated glass-making frit and fed to a melter at 1150°C. Process control and product quality are ensured by proper control of the melter feed composition. Algorithms have been developed to predict the processability of the melt and the durability of the final glass based on this feed composition. To test these algorithms, an actual radioactive waste contained in a shielded facility at SRS was analyzed and a frit composition formulated using a simple computer spreadsheet which contained the algorithms. This frit was then mixed with the waste and the resulting slurry fed to a research scale joule-heated melter operated remotely. Approximately 24 kg of glass were successfully prepared. This paper will describe the frit formulation, the vitrification process, and the glass durability.
The Product Composition Control System (PCCS) has been developed to ensure that the wasteforms produced by the Defense Waste Processing Facility (DWPF) at the Savannah River Site (SRS) will satisfy the regulatory and processing criteria that will be imposed. The PCCS provides rigorous, statistically-defensible management of a noisy, multivariate system subject to multiple constraints. The system has been successfully tested and has been used to control the production of the first two melter feed batches during DWPF Chemical Runs. These operations will demonstrate the viability of the DWPF process. This paper provides a brief discussion of the technical foundation for the statistical process control algorithms incorporated into PCCS, and describes the results obtained and lessons learned from DWPF Cold Chemical Run operations. The DWPF will immobilize approximately 130 million liters of high-level nuclear waste currently stored at the Site in 51 carbon steel tanks. Waste handling operations separate this waste into highly radioactive sludge and precipitate streams and less radioactive water soluble salts. (In a separate facility, soluble salts are disposed of as low-level waste in a mixture of cement slag, and flyash.) In DWPF, the precipitate steam (Precipitate Hydrolysis Aqueous or PHA) is blended with the insoluble sludge and ground glass frit to produce melter feed slurry which is continuously fed to the DWPF melter. The melter produces a molten borosilicate glass which is poured into stainless steel canisters for cooling and, ultimately, shipment to and storage in a geologic repository.
Radioactive high level waste (HLW) at the Savannah River Site (SRS) has successfully been vitrified into borosilicate glass in the Defense Waste Processing Facility (DWPF) since 1996. Vitrification requires stringent product/process (P/P) constraints since the glass cannot be reworked once it is poured into ten foot tall by two foot diameter canisters. A unique "feed forward" statistical process control (SPC) was developed for this control rather than statistical quality control (SQC). In SPC, the feed composition to the DWPF melter is controlled prior to vitrification. In SQC, the glass product would be sampled after it is vitrified. Individual glass property-composition models form the basis for the "feed forward" SPC. The models transform constraints on the melt and glass properties into constraints on the feed composition going to the melter in order to guarantee, at the 95% confidence level, that the feed will be processable and that the durability of the resulting waste form will be acceptable to a geologic repository.
Radioactive high level waste (HLW) at the Savannah River Site (SRS) has successfully been vitrified into borosilicate glass in the DWPF since 1996. Vitrification requires stringent product/process (P/P) constraints since the glass cannot be reworked once it has been poured into ten foot tall by two foot diameter canisters. A unique "feed forward" statistical process control (SPC) was developed for this control rather than relying on statistical quality control (SQC). In SPC, the feed composition to the DWPF melter is controlled prior to vitrification. In SQC, the glass product would be sampled after it is vitrified. Individual glass property-composition models form the basis for the "feed forward" SPC. The models transform constraints on the melt and glass properties into constraints on the feed composition going to the melter in order to determine, at the 95% confidence level, that the feed will be processable and that the durability of the resulting waste form will be acceptable to a geologic repository.
The DWPF at the Department of Energy's (DOE) Savannah River Site (SRS) began radioactive operations in December of 1995. The High Level Waste Tank Farm at SRS contains approximately thirty three million gallons of salt, supernate, and insoluble sludge wastes accumulated during more than three decades of weapons manufacture. In the DWPF, the radioactive components from this waste will ultimately be processed into a stable, borosilicate glass for long-term storage in a geological repository. The feeds to the DWPF are pretreated in a number of steps. Insoluble sludges, primarily aluminum, iron and other transition metals, are combined from several tanks, treated by caustic dissolution of aluminum and washed to remove soluble salts; these materials are removed to increase waste loading in the glass produced by the DWPF. The water soluble radioactive species in the salt and supernate, primarily cesium and actinides, are precipitated by sodium tetraphenylborate (NaTPB) or adsorbed onto sodium titanate. The resulting solids are also washed to remove excessive soluble salts before feeding to the DWPF. The soluble species removed by washing are disposed of as low level radioactive waste in a concrete form known as Saltstone. The presentation includes a brief overview of the High Level Waste system, pretreatment, and disposition of the various streams. The washed tetraphenylborate precipitates of cesium and potassium are hydrolyzed by copper catalyzed formic acid hydrolysis in the Salt Processing Cell (SPC) to yield soluble formates, boric acid, benzene and minor organic byproducts. The benzene and most of the organic byproducts are then steam stripped. The resulting aqueous hydrolysis product, including the still insoluble actinides adsorbed onto sodium titanate, is combined in the Chemical Processing Cell (CPC) with the insoluble sludge which has been treated with nitric acid and formic acid to remove mercury and to adjust the glass redox. Borosilicate glass frit is added and after assuring the melter feed meets glass quality and processing requirements, the slurry is fed to the melter producing glass which is poured into stainless steel canisters. The canisters are sealed, blasted to remove surface contamination, and welded prior to temporary storage in the Glass Waste Storage Building (GWSB). An overview of the DWPF process and its chemistry is included. The composition of the feeds is of primary importance to the DWPF. Critical factors determined by the feeds are related to safety, process design and operability, and glass quality. The Safety Analysis Report (SAR) source term, process shielding, potential for criticality, and generation of flammable gases are safety factors related to feed composition. Canister heat generation, NO(subscript x) emissions, and corrosive species are process design parameters determined by feed composition. Nitrite in the washed precipitate, glass insolubles, glass liquidus (temperature of complete melting) and glass melt viscosity are operability parameters determined by composition. And glass durability is the critical quality parameter which requires knowledge and control of the feed compositions. The basis for each of these composition related factors is presented and the system for specifying feed acceptance criteria is described. The composition, and thus the durability, of the glass is determined by the mixing ratios of sludge insolubles, aqueous hydrolysis product, and frit. The frit is a purchased raw material; naturally, its composition is essentially fixed. Also, the glass components in the aqueous hydrolysis product are essentially invariant because the cesium plus potassium to boron ratio is unity, essentially all of the water is evaporated, and the sodium titanate concentration is carefully controlled in the precipitation process. Therefore, the sludge composition is the primary source of feed variability. The combination of process and tank farm history, strategic tank samples, system waste removal plans, and process modeling which project sludge batch composition and evaluate process related parameters and glass durability is described. All the sludge batches, each of which can feed the DWPF for several years, is projected and evaluated through completion of waste removal. Finally, extensive sludge characterization through sampling and analysis is combined with small scale testing in the Shielded Cells of the Savannah River Technology Center (SRTC) to assure the sludge batch meets all the feed acceptance criteria.
The Defense Waste Processing Facility (DWPF) at the Savannah River Site (SRS) in Aiken, South Carolina, will be used to immobilize the approximately 130 million liters of high-level nuclear waste currently stored at the site in 51 carbon steel tanks. Waste handling operations separate this waste into highly radioactive insoluble sludge and precipitate and less radioactive water soluble salts. (In a separate facility, the soluble salts are disposed of as low-level waste in a mixture of cement, slag, and flyash.) In DWPF, precipitate (PHA) is blended with insoluble sludge and ground glass tit to produce melter feed slurry which is continuously fed to the DWPF melter. The melter produces a molten borosilicate glass which is poured into stainless steel canisters for cooling and, ultimately, shipment to and storage in a geologic repository. The repository requires that the glass wasteform be resistant to leaching by underground water that might contact it. In addition, there are processing constraints on melt viscosity, liquidus temperature, and waste solubility.
The Defense Waste Processing Facility (DWPF) will immobilize Savannah River Site High Level Waste as a durable borosilicate glass for permanent disposal in a repository. The DWPF will be controlled based on glass composition. The following discussion is a preliminary analysis of the capability of the laboratory methods that can be used to control the glass composition, and the relationships between glass durability and glass properties important to glass melting. The glass durability and processing properties will be controlled by controlling the chemical composition of the glass. The glass composition will be controlled by control of the melter feed transferred from the Slurry Mix Evaporator (SME) to the Melter Feed Tank (MFT). During cold runs, tests will be conducted to demonstrate the chemical equivalence of glass sampled from the pour stream and glass removed from cooled canisters. In similar tests, the compositions of glass produced from slurries sampled from the SME and MFT will be compared to final product glass to determine the statistical relationships between melter feed and glass product. The total error is the combination of those associated with homogeneity in the SME or MFT, sampling, preparation of samples for analysis, instrument calibration, analysis, and the composition/property model. This study investigated the sensitivity of estimation of property data to the combination of variations from sampling through analysis. In this or a similar manner, the need for routine glass product sampling will be minimized, and glass product characteristics will be assured before the melter feed is committed to the melter.
